1. Field of the Invention
The invention relates to an injector for injecting fuel into the combustion chambers of internal combustion engines. Injection systems for direct-injection internal combustion engines must now meet ever-increasing demands. For instance, it is demanded that the injection pressure and injection quantity be capable of being defined independently of one another for every operating point of the engine, so that there is one additional degree of freedom for mixture formation. At the onset of injection, the injection quantity should be as slight as possible, for the sake of the ignition delay that ensues between the onset of injection and the onset of combustion. These demands are currently met in reservoir-type injection systems (common rails) with pre-injection and main injection phases.
2. Description of the Prior Art
German Patent Disclosure DE 198 35 494 A1 relates to a unit fuel injector. It serves to deliver fuel to the combustion chamber of direct-injection internal combustion engines. A pump unit is furnished for building up an injection pressure and injecting the fuel into the combustion chamber via an injection nozzle. This is embodied with a control unit together with a control valve that is embodied as an outward-opening A-valve and with a valve actuation unit. With the valve actuation unit, the pressure buildup in the pump unit is controlled. To create a unit fuel injector with a control unit that is simple in structure, small in size, and in particular has a short response time, the valve actuation unit is embodied as a piezoelectric actuator.
From German Patent DE 37 28 817 C2, a fuel injection pump for an internal combustion engine is known. The fuel injection pump includes a control valve member comprising a valve shaft that forms a guide sleeve and slides in a conduit and a valve head connected to the valve shaft and oriented toward the actuating device. The sealing face of the valve head is embodied to cooperate with the face of the control bore that forms the valve seat. The valve shaft, on its circumference, has a recess whose axial length extends from the orifice of the fuel supply line to the beginning of the sealing face on the valve head but cooperates with the valve seat. A face exposed to the pressure of the fuel supply line is embodied in the recess that is equal in size to a face of the valve head exposed, in the closed state of the control valve, to the pressure of the fuel supply line. The result, in the closed state of the valve, is a state of pressure equilibrium. The guide sleeve receives a spring that urges the control valve toward its open position.
In injection systems used previously the triangular stroke course desired often proves to be quite bulky, since when the control valve is opened the valve springs open, but with the version proposed by the invention for a control part in an injector for injecting fuel, the ensuing course of injection pressure can be better adapted to the combustion.
By connecting a flat element embodied as an annular throttle downstream, the appropriate flow quantity upon opening of the control part can be specified with extreme precision. If the annular throttle includes a conical face and a cylindrical part, then by way of specifying the cone angle at the truncated cone and the length of the shoulder on the control part embodied as a truncated cone, an adjustment of the pressure course can be done by way of the stroke of the control part.
The cylindrical portion of the annular throttle element is quite simple to manufacture technically on a rotationally symmetrical component. The cylindrical part of the annular gap throttle can be minimized except for a control edge, whose underside is adjoinedxe2x80x94in the downstream directionxe2x80x94by the truncated cone of the annular throttle. Minimizing the cylindrical portion of the annular throttle element to a control edge would lead to further shortening and thus economy in terms of structural length of the injector and injector housing. The vertical up and down motion for opening and closing the valve chamber is impressed on the injector by way of a separately actuatable valve control unit, by whose opening a control chamber acting on the control part is pressure-relieved.
With the embodiment proposed by the invention of the throttle element with a truncated cone, the pressure course during injection can be adapted to the course of combustion. The injection onset, injection course, and atomization of the fuel affect the fuel consumption of an internal combustion engine and hence pollutant emissions considerably. A late injection reduces the NOx emissions as a consequence of low process temperatures. An overly late injection increases the HC emissions and fuel consumption, as well as the expulsion of soot at higher loads. A deviation of the injection onset from the desired value by only one degree of crankshaft angle can increase the NOx emissions by up to 5%. An injection onset that is too early by 2xc2x0 of crankshaft angle can lead to an increase in the peak cylinder pressure of 10 bar, while a shift toward xe2x80x9clatexe2x80x9d by 2xc2x0 of crankshaft angle can increase the exhaust gas temperature by 20xc2x0 C. This high sensitivity demands a precisely set injection onset and requires that the previously calculated course of injection be adhered to. The course of injection is defined by the fuel quantity, which varies during an injection cycle (that is, from the onset to the end of an injection). The injection course determines the fuel mass pumped during the ignition delay (between the onset of injection and the onset of combustion). Furthermore, it also affects the distribution of the fuel in the combustion chamber and thus the utilization of the air. The injection course must rise slowly so that only little fuel will be injected in the ignition delay. At the onset of combustion, this fuel burns forcefully (premixed combustion), which has an unfavorable effect on noise and NOx emissions. At the end, the course of injection must drop off sharply, to prevent poorly atomized fuel in the final phase from leading to high hydrocarbon and soot emissions and increased fuel consumption.
With the injector proposed according to the invention, with an annular throttle element downstream of the seat diameter of the control part, the fuel flow rate through the nozzle can be adapted more precisely to the fuel flow rate specified by the course of combustion, for the sake of achieving the most homogeneous possible combustion.